Divorce in The Sims 3
I had an odd experience with divorce in Sims 3 over the weekend. I didn't realize that Sims can divorce themselves! This is one of the few things that a Sim will do on their own, if they are in your playable family. Non-playable Sims will spontaneously get married, have babies, get divorced, take new jobs, etc. But your playable Sims will as a rule not make any life-changing decisions on their own.
Furthermore, I don't believe this is how it worked in The Sims or The Sims 2. You had a much easier time cheating on your spouse in The Sims, of course - as long as your spouse wasn't in the same room, you could do whatever you want. In The Sims 2, your spouse could find out what was happening as long as they were on the same lot when you engaged in hanky panky.
In either case, I don't believe the cheated-upon Sim ever took action on their own. Or maybe I was just the exception in having a pair of Sims with the most unhappy marriage of all time!
Over the weekend, I took one of my married Sims and sent her to France. Once there, she cheated on her husband with the Special Merchant. When we returned to Barnacle Bay, I discovered that she no longer had a husband: he was now her "Ex-Husband."
That's right: not only did her husband spontaneously divorce her, but he found out she was cheating when she was in another world!
I suspect the paparazzi might have something to do with this. A photographer snuck into their room and captured the happy couple cuddling on the bed. At about the same time, my Sim's reputation changed to "cheater." I'm not sure if one caused the other, or if it's just that both things happened at the same time. At any rate, word got back to her husband at home!
Out of curiosity, I kept them both in the same house. Their relationship was pretty far in the red when she came home from France, at about 15%. As time went on, every social interaction they shared only made it worse. And the worse it got, the more likely it was that they would have a negative interaction (like nagging or insulting).
In the end, I had to move her ex-husband out, because I was having a glitch in the game that I wasn't able to resolve. It's too bad, because I had been toying with the idea of trying to get them back together. I guess we'll never know!